The present invention relates to the structure of a CCD (Charge Coupled Device) package and the method for assembling it. Currently, the CCD packages conventionally used are classified into two general types, plastic packages and ceramic packages. Different processes are used to assemble each type of package. To help better understand the current art in the assembly of CCD packages, each process will be described below in detail.
The first process to be described is the assembly of a ceramic package. Ceramic packages are mainly classified into CERDIP (CERAMIC+DIP) type packages and multilayer type packages. Herein, only the process for assembling a multilayer type ceramic package will be described. FIGS. 1a to 1d help describe this process, while FIG. 1e shows the finished ceramic package.
As a first step, several sheets are prepared by using a powder obtained by mixing an Al.sub.2 O.sub.3 compound with certain additives. On these sheets, patterns are formed which are to be used in the respective layers of the package. In the forming of these patterns, metal pads of leads which are to be wire-bonded with the bonding pads of a die are also formed. Together with a lead frame previously prepared, the sheets are stacked in turn and then subjected to sintering, so as to form a package 1 of the desired shape as shown in FIG. 1a. The ceramic package 1 takes its shape through the use of three layers, that is a bottom layer 2, a middle 3 and a top layer 4. Of course, the ceramic package 1 may have more layers.
Next, die bonding is carried out, as shown in FIG. 1b, so as to attach a die (or a chip) 5, which may be a CCD element, on a paddle of the lead frame (not shown) which has been fixed to the ceramic package 1. Then, wires 6 are bonded to the bonding pads (not shown) of the die 5 and each corresponding lead of the lead frame, so as to connect them as shown in FIG. 1c. In order to cover the open portion of the package 1, a glass layer 7 is then formed, as shown in FIG. 1d. The glass layer 7 functions to transmit light to a light receiving region of the CCD element. Thereafter, leads are attached to predetermined positions on both sides of the package 1. FIG. 1e reveals the structure of the finally obtained step-shaped package.
A hollow-type plastic package is assembled in a similar manner. FIGS. 2a through 2f will be used to help illustrate this procedure, while FIG. 2g shows the finished hollow-type plastic package.
Wafer 8 in FIG. 2a is subjected to a dicing process and divided into chips 9, also known as "dies." The dicing process may be accomplished by several means, including a chemical method using acetic acid or fluoroacetic acid, and a scribing method using a diamond cutter.
After the dicing process, die bonding is carried out, so as to attach the die 9 to a paddle 11 of a previously prepared lead frame 10. The lead frame 10, shown in FIG. 2b, also comprises side rails 12, lead fixing means or dambers 14, support bars 15, locking holes 16, and leads 13. Each of the leads 13 includes an inner lead 13a and an outer lead 13b. The lead fixing means or dambers 14 act to maintain a uniform space between adjacent leads 13 and the lead frame 10.
To accomplish the die bonding, an eutectic alloy process may be used. To do so, a gold-antimony alloy is thinly coated on the paddle 11. With the die 9 then seated on the coated gold-antimony alloy layer, the paddle 11 is heated. Through this heating, the gold-antimony alloy is eutectic-welded into the silicon material of the die 9. The heating temperature may be as high as 300.degree. C. to 400.degree. C., depending upon the kind of solder materials used. In order to avoid the oxidation of the die 9 or the paddle 11 under this high temperature, the heating is generally carried out in an atmosphere of inert gas such as nitrogen.
Other die bonding processes may be used, including use of a conductive epoxy based adhesive, a soldering process using a conventional Pb-Sn solder, and a glass soldering process. In the glass process, soldering glass is disposed on a substrate and melted at a temperature of about 500.degree. C. to 600.degree. C. A ceramic die pack is then pressure bonded on the melted soldering glass.
After the die bonding, a wire bonding process is performed, as shown in FIG. 2c. The wire bonding process connects bonding pads 17 of the die 9 with inner leads 13a of the lead frame 10 by means of wires 37. The wires 37 are usually made of aluminum or gold. To accomplish the wire bonding process, one may use a thermal pressure bonding process, an ultrasonic process, a soldering process, a laser process or an electron beam process. The thermal pressure bonding process and the ultrasonic process are preferred in actual practice.
As shown in FIG. 2d, guide blocks 18 are then formed on the die 9 around the both sides of the light receiving region 38 of the die 9. A glass lid 19 is then placed on top of the guide blocks 18, thereby covering the light receiving region 38.
Next, the lead frame 10, to which the die 9 and the wires 37 have been bonded, is positioned in a previously prepared die mold. An epoxy molding compound is then charged into the mold and die molded. This molding encases the die except for the light receiving region 38, which is protected by the guide blocks 18 and the glass lid 19. In this manner, a hollow-type plastic package 20 is formed, as shown in FIG. 2e.
As shown in FIG. 2f, a trimming process is then performed to remove the lead fixing means or dambers 14. Thereafter, a forming process is carried out, so as to form the outer leads 13b into a preferred shape. After the forming process, the outer leads 13b may have a gull-wing shape or a J-bend shape. The completed plastic hollow-type package is shown in FIG. 2g.
The above-mentioned conventional methods of assembling CCD packages, while capable of creating useable packages, have the following disadvantages.
First, the packages manufactured by both of the above conventional methods are relatively heavy and bulky so that when they are used in camcorders and still cameras, they occupy a large volume in the interior of these products, thereby increasing the product's size and weight.
Second, ceramic packages are expensive to manufacture, thereby increasing the cost of the products in which they are used.
Third, in the case of hollow-type plastic packages in which glass lids are laid over the chips, the glass lids have a tendency to shift out of position when the die molding is performed. Consequently, the rate of poor products is increased and productivity is decreased.
Therefore, it is an object of the invention to overcome the above-mentioned disadvantages encountered in the prior art and to provide a method for assembling a CCD package by using a tape (a flexible insulated backing with conductive strips for leads and interconnect surfaces for mating with the terminals of the chip) designed for TAB (Tape Automated Bonding) having leads and a paddle for attaching a die. Through this method, packages of a light, laminated and simple structure can be obtained, thereby decreasing manufacturing costs and simplifying the assembly process.